1. Field of the Invention
The present invention generally relates to the manufacturing of optical fibers. More specifically, the present invention relates to a method and a system for coloring optical fibers.
2. Description of the Related Art
An optical fiber generally comprises a core surrounded by a cladding (hereinafter both collectively referred to as “optical waveguide”), said core and cladding being preferably made of glass, and at least one exterior coating.
In some instance the exterior coating is one (single coating), in others the exterior coatings are two (double coating). In the latter case, the exterior coating directly contacting the optical waveguide is called “first coating” or “primary coating” and the exterior coating overlying the first one is called “second coating” or “secondary coating”. Typically, said first and second coatings are made of polymer material.
In telecommunications applications, many optical fibers are often bundled together, for example in multiple individual optical fiber strands that are packaged to form a ribbon or a cable. It is important, for the operators dealing with the installation and maintenance, to be able to distinguish the individual optical fibers in the ribbon or cable.
To aid identification and distinction of the optical fibers when many of them are packaged together, color coding of the optical fibers is adopted.
The coloring of the optical fibers is made during the fiber manufacturing, after one or more coating layers are applied to the drawn fiber. For example, one of the fiber coatings, e.g. the secondary coating can be colored.
A pre-colored coating material is applied onto the optical fiber, such pre-colored material being prepared by mixing a non-colored coating material with a colorant, typically a polymeric composition comprising a pigment before the application onto the fiber.
For example, in EP 1640347 an apparatus and method for color-coating an optical fiber are described, wherein mixing of the color concentrate with secondary coating is performed directly in the coating die. One or more color reservoirs are provided. The reservoirs are connected via relevant feed lines, to the coating die, which also receives, from a coating feed line, the secondary coating; a flow controller, like a positive-displacement pump, a reciprocating pump, a metering pump, a mass-flow controller, controls the flow of the delivered color concentrate. The speed of the flow controller is controlled or adjusted to be proportionate to the draw speed.
EP 354 289 discloses a method and an apparatus for the on-line, “on-the-draw” application onto an optical fiber of a two-package coating system. A first and a second liquid components, e.g. two liquid resins, a resin and a curing catalyst therefor, a resin and a modifier therefor such as a dye, are continuously mixed under controlled conditions and thereafter directly supplied to a liquid coater as a homogeneous liquid blend. The two liquid components are drawn from respective reservoirs through respective supply lines and precision positive displacement metering pumps, and are fed to a static mixer, which mixes the two liquid components to form a mixture continuously supplied to a coater. The rate of mixing and supply of the reactive liquid mixture to the coater is controlled by feedback from a pressure transducer attached to the coater. As the mixture is depleted by application to the fiber, the pressure drop in coater is sensed by the transducer and a feedback signal is transmitted to a programmable electronic flow controller.
WO 01/60757 describes a method for producing optical fibers. A coating composition is introduced into a liquid conduit via an inlet line. The colorant, either alone or in a suitable carrier, is introduced into the liquid conduit via a line. After introduction, the rate of flow of the coating composition through the liquid conduit and into a mixing zone is metered by a control system comprising a flow meter, a flow indicator controller and a valve controllable by the flow indicator controller. A second control system is provided to meter the colorant into the mixing zone. The colorant control system includes a colorant flow meter, a colorant flow indicator controller and a valve controllable by the colorant flow indicator controller. Based upon the flow rate of the coating composition provided by the flow meter, the ratio controller sends a signal to the appropriate colorant flow indicator controllers to affect the addition of specific colors, at certain flow rates. The colorants can be added to the coating composition at any point upstream of the mixing zone outlet. The mixing zone can be any environment capable of mixing the stock coating composition and colorants (static mixer being preferred). The mixing tank is provided with an inlet for admitting coating composition and an inlet for admitting colorant. The inlet(s) may be located at any position(s) upstream of the mixing zone. After the colorants are added and thoroughly mixed in the tank, a finished coating composition is provided. The finished coating composition exits the mixing tank, and is pumped through the liquid conduit to the device that applies the coating composition onto the optical fibers, commonly referred to as a draw tower. Between the tank and the draw tower, several components are optionally provided.